In the reconditioning of spent nuclear fuels, the fuel (uranium and plutonium oxide) and the fission products are dissolved in nitric acid. After filtration, the addition of nitric acid or water, and concentration, the solution is fed to a liquid-liquid-counterflow-extraction process known as the PUREX process for the separation of uranium, plutonium and fission products (cf. Kerntechnik, 1978, No. 2, pages 77-79). In this extraction process tri-n-butylphosphate (TBP) is the extraction agent in the form of an organic solution of TBP in kerosene for uranium and plutonium (designated below by the chemical symbols U and Pu).